1. Field of the Invention
The present invention concerns in general terms a method of obtaining a gain function in transmission mode. More particularly, the present invention relates to a method of obtaining an antenna gain in transmission mode for a base station in a mobile telecommunication system. It makes it possible to obtain an antenna gain in transmission mode from an antenna gain in reception mode.
2. Discussion of the Background
The formation of channels or the elimination of interfering signals is well known in the field of narrow-band antenna processing. Each uses an array of antennae, generally linear and uniform (that is to say with a constant pitch) and a signal weighting module. More precisely, if it is wished to form a channel in reception mode, the signals received by the different antennae are weighted by a set of complex coefficients before being added. Conversely, if it is wished to form a channel in transmission mode, the signal to be transmitted is weighted by a set of complex coefficients and the signals thus obtained are transmitted by the different antennae.
FIG. 1 illustrates a known device for obtaining an antenna gain in transmission and reception. The device comprises an array of antennae (100),(101), . . . ,(10N-1), a transmission weighting module (11) and a reception weighting module (15). The signals received by the different antennae, (xi), i=0 . . . N-1 are weighted at (130),(131), . . . ,(13N-1) by a set of complex coefficients (bui), i=0, . . . , N-1 before being added at (14) in order to give a signal Ru. Conversely, a signal to be transmitted Sd is weighted as (120),(121) . . . ,(12N-1) by a set of complex coefficients (bdi), i=0, . . . , N-1, before being transmitted by the different antennae.
If the vector of the received signals and the vector of the weighting coefficients are respectively denoted {overscore (x)}=(x0,x1, . . . ,xN-1)T and {overscore (bu)}=(bu0,bu1, . . . ,buN-1)T, it is possible to write:
Ru={overscore (buT.x)}xe2x80x83xe2x80x83(1)
The complex gain (or the complex gain function of the antenna) in reception mode can be written:                               G          ⁡                      (                                                            b                  u                                _                            ,              θ                        )                          =                                                                              b                  u                  T                                ·                            _                        ⁢                                          e                uθ                            _                                =                                    ∑                              i                =                0                                            N                -                1                                      ⁢                          xe2x80x83                        ⁢                                          b                ui                            ·                              exp                ⁡                                  (                                                            -                      j                                        ⁢                                          xe2x80x83                                        ⁢                                          ϕ                      i                                                        )                                                                                        (        2        )            
where {overscore (euxcex8)} represents the vector {overscore (x)} corresponding to a flat wave arriving at an angle of incidence xcex8, and
xcfx86i=(2xcfx80d/xcex).i. sin (xcex8)=(2xcfx80dƒ/c).i. sin (xcex8)xe2x80x83xe2x80x83(3)
is the difference in operation between consecutive antennae for a uniform linear array with a pitch d, xcex and ƒ being respectively the wavelength and the frequency of the flat wave in question;
and
xcfx86i=2xcfx80Rxcex94xcex8/xcex sin (xcex8xe2x88x92xcex8i)=2xcfx80Rƒxcex94xcex8/c sin (xcex8xe2x88x92xcex8i)xe2x80x83xe2x80x83(4)
for a circular array where xcex8i is the angle between a reference axis and the normal to the antenna of index i, R the radius of curvature of the array, xcex94xcex8 is the angular difference between two consecutive antennae in the array.
Likewise, the complex gain (or the complex gain function) in transmission can be written:                               G          ⁡                      (                                                            b                  d                                _                            ,              θ                        )                          =                                                                              b                  d                                _                            T                        ·                                          e                                  d                  ⁢                                      xe2x80x83                                    ⁢                  θ                                            _                                =                                    ∑                              i                =                0                                            N                -                1                                      ⁢                          xe2x80x83                        ⁢                                          b                di                            ·                              exp                ⁡                                  (                                      j                    ⁢                                          xe2x80x83                                        ⁢                                          ϕ                      i                                                        )                                                                                        (        5        )            
with the same conventions as those adopted above and where {overscore (edxcex8)} is the vector {overscore (x)} corresponding to a flat wave transmitted in the direction xcex8. The weighting vectors in reception and transmission mode will be called respectively {overscore (bu)} and {overscore (bd)}.
When the array of antennae is functioning in reception mode at a given frequency, different known methods, notably the Wiener filtering method, make it possible to determine the weighting vector {overscore (bu)} which maximises the signal to noise ratio. In a mobile telecommunications system, the array of antennae of a base station receives signals transmitted by a plurality of mobile terminals. In the context of a transmission in CDMA (Code Division Multiple Access) mode, the signals transmitted by the different mobile terminals are separated by means of the use of orthogonal codes on transmission and filters adapted to these codes on reception. In practice, however, the separation of the different signals received is not perfect. For an uplink between a given mobile terminal and the base station which serves it, the criterion to be maximised is then the ratio of signal to noise plus interference, the latter being due to the signals transmitted by the other mobile terminals. Likewise, the downlink between a base station and a given mobile terminal is disturbed not only by the background noise but by the interference due to the signals transmitted by the said base station to other mobile terminals. Though it is relatively easy to optimise the weighting vector in reception mode, {overscore (bu)}, by estimating the uplink channel and the density of interference at the base station, it is quite different with regard to the optimisation of the weighting vector in transmission mode, {overscore (bd)}. This is because the estimation of the downlink channel and the density of interference cannot be made directly at the base station and a transmission of this information by the mobile terminals is necessary. However, this transmission of information consumes conveyance resources on the uplink, which can be disadvantageous, notably in the case of rapid variations in the channel transfer function, for example when the mobile terminal is moving at high speed.
The aim of the invention is to propose a method of determining the transmission weighting vector, {overscore (bd)}, optimising the ratio of signal to noise plus interference on the downlink and requiring the transmission only of a small quantity of information on the uplinks.
To this end, the invention is defined by a method of obtaining a transmission gain function by means of an array of antennae and a weighting of the signals received or to be transmitted by vectors ({overscore (b)}) of N complex coefficients, referred to as weighting vectors, N being the number of antennae in the array, the array transmitting, to a telecommunication terminal on a transmission channel, referred to as the downlink channel, a downlink transmission signal (Sd) and the said terminal transmitting to the said array on a transmission channel, referred to as the uplink channel, an uplink transmission signal (Su), the said uplink channel being disturbed by a first isotropic noise (N) and/or a first directional noise, referred to as the uplink interference (Iu), the said downlink channel being disturbed by a second isotropic noise (Nxe2x80x2) and/or a second directional noise, referred to as the downlink interference (Id), a first weighting vector ({overscore (b)}u) having been determined in order to maximise, on reception by the array, the ratio (C/(I+N))u of the received signal coming from the said terminal to the noise plus interference disturbing the said uplink channel, a second weighting vector ({overscore (b)}d) maximising, on reception by the terminal, the ratio (C/(I+N))d of the received signal coming from the network to the noise plus interference disturbing the downlink channel, is calculated from the said first weighting vector in the form of a matrix product comprising a first noise matrix (Du) which is a function of the power of the first isotropic noise and/or the power of the first directional noise and a second noise matrix (Dd) which is a function of the power of the second isotropic noise and/or the power of the second directional noise.
According to one embodiment, the first weighting vector ({overscore (b)}u) is obtained for a first working frequency (ƒu) of the array and the second weighting vector ({overscore (b)}d) is obtained for a second working frequency (ƒd) of the array.
Advantageously, a gain function being represented by a vector, referred to as the gain vector ({overscore (G)}), of M complex samples of the said gain function taken in M distinct directions (k), the said gain vector being expressed as the product of a weighting vector ({overscore (b)}) and a transformation matrix dependent on the working frequency of the array, the second weighting vector {overscore (b)}d is obtained from the first weighting vector {overscore (b)}u by:
{overscore (b)}d=Hd+.Dd2.Du2.Hu{overscore (b)}u
where Hu is the transformation matrix at the frequency (ƒu), Hd+ is the pseudo-inverse matrix of the matrix Hd, the transformation matrix at the frequency (ƒd), Du is the first noise matrix and Dd is the second noise matrix.
The first noise matrix can be expressed as a diagonal matrix of size Mxc3x97M and of components {square root over ("sgr"uk2+xcex3uN0/Iu)} where "sgr"uk2 is the power of the uplink interference in the direction k, Iu is the total power of the uplink interference, and N0 is the power of the first isotropic noise, xcex3u=1/∥{overscore (G)}u∥2 where {overscore (G)}u=Hu{overscore (b)}u.
In a similar manner, the second noise matrix can be expressed as a diagonal matrix of size Mxc3x97M and of components {square root over ("sgr"dk2+xcex3dNxe2x80x20/Id)} where "sgr"dk2 is the power of the downlink interference in the direction k, Nxe2x80x20 is the power of the second isotropic noise, xcex3d=1/∥{overscore (C)}d∥2 where {overscore (C)}d is a vector consisting of the samples of the function of the transfer function of the downlink channel taken in the said M directions and Id is the total power of the downlink interference.
If the array transmits on a plurality of downlink channels a plurality of transmission signals to a plurality of telecommunication terminals and receives from them a plurality of transmission signals transmitted on a plurality of uplink channels and if each downlink channel j relating to a terminal j of the said plurality is associated with a second weighting vector {overscore (b)}d(j), the second noise matrix relating to the downlink channel j is a diagonal matrix of size Mxc3x97M and of components {square root over ("sgr"dk2(j)+xcex3d(j).Nxe2x80x20/Id(j))} where "sgr"dk2(j) is the power of the downlink interference for the downlink channel j in the direction k, xcex3d(j) is a coefficient characterising the power transfer on the downlink channel j, Nxe2x80x20 is the power of the second isotropic noise, and Id is the total power of the downlink interference.
The downlink interference power in the direction k, "sgr"dk2 (j), can be estimated according to the power of the transmitted signals Sd(jxe2x80x2) on the downlink channels jxe2x80x2 distinct from j by:                     I        d            ⁢              (        j        )              ·                  σ        dk        2            ⁢              (        j        )              =                    β        d            ⁢              (        j        )              ·                  ∑                  j          ≠          j                    ⁢                                    "LeftBracketingBar"                                          S                d                            ⁢                              (                                  j                  xe2x80x2                                )                                      "RightBracketingBar"                    2                ·                              "LeftBracketingBar"                                          g                dk                            ⁢                              (                                  j                  xe2x80x2                                )                                      "RightBracketingBar"                    2                    
where xcex2d(j) is an orthogonality coefficient of the downlink channel j and gdk(jxe2x80x2) is the kth coefficient of the gain vector {overscore (G)}d(jxe2x80x2)=Hd{overscore (b)}d(jxe2x80x2) relating to the downlink channel jxe2x80x2.
Advantageously, the said coefficient xcex2d(j) is estimated from a coefficient characterising the orthogonality of the uplink channel j.
The coefficient xcex3d(j) can be transmitted to the array by the terminal j on the associated uplink channel.
The invention is also defined by a device adapted to implement the method which has just been disclosed.